A deep look into the core of young clusters*
II. 
Orionis
1
Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 - La Laguna, Tenerife, Spain e-mail: bouy@iac.es
2
Laboratorio de Astrofísica Estelar y Exoplanetas, LAEX-CAB (INTA-CSIC), PO BOX 78, 28691, Villanueva de la Cañada, Madrid, Spain
3
University of Central Florida, Department of Physics, PO Box 162385, Orlando, FL 32816-2385, USA
4
European Southern Observatory, Karl Schwartzschild Str. 2, 85748 Garching bei München, Germany
5
Gemini Observatory, Southern Operations Center, Association of Universities for Research in Astronomy, Inc., Casilla 603, La Serena, Chile
6
Astronomy Department, University of California, Berkeley, CA 94720, USA
7
National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
8
European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
9
Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
Received:
25
May
2009
Accepted:
2
July
2009
Context. Over the past years, the Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function and other properties of low mass cluster members.
Aims. In the continuity of our previous studies of young associations cores, we search for ultracool members and new multiple systems within the central 5
3 (≈0.6 pc) of the cluster.
Methods. We obtained deep seeing limited J, Ks-band images of the 53 central part of the cluster with NTT/SofI and H-band images with CAHA/Omega2000. These images were complemented by multi-conjugate adaptive optics (MCAO) H and Ks images of the 15 central region of the Orionis cluster obtained with the prototype MCAO facility MAD at the VLT. The direct vicinity of the massive Ori O8III-star was probed using NACO/SDI at the VLT. Finally, we also retrieved Spitzer IRAC images of the same area and used archival Subaru Suprime-Cam and CFHT CFHT12K i-band images.
Results. We report the detection of 9 new member candidates selected from optical and near-IR color-color and color-magnitude diagrams and 7 previously known members. The high spatial resolution images resolve 3 new visual multiple systems. Two of them are most likely not members of the association. The third one is made of a brown dwarf candidate companion to the F8V star HD 36861C. The simultaneous differential images allow us to rule out the presence of visual companions more massive than 
in the range 1–2.5´´, and 
in the range 0
5–2.5´´.
Key words: stars: evolution / stars: formation / stars: general / stars: low mass, brown dwarfs / techniques: high angular resolution
Based on observations made at the ESO La Silla and Paranal Observatory under programmes 082.C-0724, 080.D-0532(, 67.C-0042, 074.C-0084, and 074.C-0628, on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC) and at the Calar Alto Observatory, on observational data obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii, on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, and on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
© ESO, 2009